The present invention is generally related to radiotelephones, and more particularly to digitally frequency-locking a cellular telephone to the received signal.
In some countries, cellular telephones are required to meet a maximum transmitter frequency error specification. In order to maintain the transmitter frequency within the maximum transmitter frequency error specifications, such cellular telephones currently use an analog frequency control circuit which compares the received signal frequency to the cellular telephone reference oscillator to develop an analog correction voltage. The analog correction voltage is applied to the cellular telephone reference oscillator to frequency lock it to the received signal frequency, which is very accurately maintained by the base station radio. However, such prior analog frequency control circuits necessitate digital-to-analog conversion circuitry and analog comparators thereby incurring substantial additional costs.